1. Field of the Invention
The present invention relates to a zoom lens system.
2. Related Background Art
There has been proposed a lot of so-called positive leading type four-lens-group zoom lens systems composed of, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power, and carrying out zooming by varying distances between lens groups in Japanese Patent Application Laid-Open Nos. 8-248319 and 2002-323655.
A characteristic of a zoom lens system of this type is to be able to obtain a high zoom ratio by actively varying a distance between the first lens group and the second lens group, and a distance between the second lens group and the third lens group. Moreover, in a zoom lens system of this type, by slightly changing a distance between the third lens group and the fourth lens group, both of which have positive refractive power, variation in curvature of field liable to be a problem in a high zoom ratio zoom lens can be corrected, so that excellent optical performance can be accomplished.
However, in such conventional zoom lens system mentioned above, the fact that large aberrations generated upon zooming can be corrected by slightly changing the distance between the third lens group and the fourth lens group means that variation in the very distances have a significantly large influence (hereinafter called “distance variation sensitivity”) on deterioration in optical performance, and that decentering between the third lens group and the fourth lens group have a considerably large influence (hereinafter called “decentering sensitivity”) on deterioration in optical performance. Accordingly, high precision is to be required upon manufacturing the zoom lens system.
Moreover, in such conventional zoom lens system mentioned above, since zooming is carried out by varying distances before and after the second lens group having large refractive power, decentering sensitivity of the second lens group also becomes high. This also requires high precision upon manufacturing the zoom lens system.